Magnetic proximity switches, also known as limit switches, are commonly used for linear position sensing. Typically, magnetic proximity switches include a target and a sensor, the sensor being coupled to a switching circuit having two leaf portions, a stationary portion and a movable portion hermetically sealed within a glass enclosure that is itself contained within a switch body. When the target, which generally includes a permanent magnet contained within a housing, passes within a predetermined range of the sensor, the magnetic flux generated by the target magnet causes the movable leaf to contact the stationary leaf, thereby closing the switch.
The ability of the sensor to detect the position of the magnetic target without the need for physical contact allows the magnetic proximity switch to be used in applications where the target is contained inside a sealed housing and the sensor is located outside of the housing. For example, a main steam isolation valve (MSIV) is a gate valve that is placed between a turbine and a boiler in a nuclear reactor system. The MSIV is used to cut off steam flow between the boiler and the turbine if radioactive material leaks into the steam supply. Because of the need to securely contain the radioactive material, the MSIV is housed within a secure pressure vessel. A magnetic target coupled to the valve stem of the gate of the MSIV allows a sensor positioned outside of the pressure vessel to detect whether the gate has reached a particular position, such as a position in which the MSIV is closed.
In applications like the MSIV described above, the magnetic target and the sensor are usually separated by a relatively large distance. In such cases, the magnetic target frequently includes a cylindrical radially-magnetized samarium cobalt (SmCo) magnet 10. It is recognized that such a SmCo magnet 10 has a strong magnetic flux field 12, illustrated in FIG. 1, having the general shape of a partial ellipse that intersects the top and bottom surface of the SmCo magnet 10. The partial ellipse has a major axis 14 normal to the longitudinal axis 16 of the magnet 10 such that the ellipse is elongated away from the longitudinal axis 16. Because a sensor 18 detects components of the flux field 12 that perpendicularly intersect a planar detection surface 20 of the sensor 18, one skilled in the art will appreciate that the elongated shape of the flux field 12 of the SmCo magnet 10 enables the sensor 18 to detect the SmCo magnet 10 when the SmCo magnet 10 is relatively far from the longitudinal axis 16.
A further feature of the radially-magnetized SmCo magnet 10 is that the flux field is substantially uniform about the longitudinal axis of the magnet. Consequently, if a sensor detects the flux field at a particular point in space, the radially-magnetized SmCo magnet can be rotated about its longitudinal axis without affecting the sensor's ability to detect the magnet's flux field. Such longitudinal uniformity may be desirable in applications where the magnetic target is coupled to a valve element that may rotate, such as a valve stem of an MSIV.
However, there may be significant drawbacks to the use of radially-magnetized SmCo magnets. Specifically, radially-magnetized SmCo magnets are difficult, and expensive, to manufacture. While traditional magnets are cheap and simple to produce, such magnets have relatively weak magnetic flux fields that are unable to be detected when a barrier, such as a pressure vessel wall, separates the magnet from the sensor. The flux fields of such magnets are also longitudinally non-uniform, and therefore a slight rotation of the magnet relative to a stationary sensor could render the target undetectable to the sensor. Axially-magnetized SmCo magnets are also cheaper and easier to manufacture than radially-magnetized SmCo magnets, but the elliptical flux fields of such magnets have a major axis that is parallel, not normal, to the longitudinal axis of the magnet. One skilled in the art would recognize that a sensor must be relatively close to an axially-magnetized SmCo magnet to detect such a flux field, making such a magnet unsuitable for applications that require a relatively large distance between the sensor and the target.
Therefore, there exists a need for an inexpensive and easy-to-manufacture alternative to the radially-magnetized SmCo magnet, without compromising the strength and longitudinal uniformity of the radially-magnetized SmCo magnet's flux field.